Mechanism of silver staining of histones: evidence for involvement of clustered lysine residues.

Pretreatment of histones with formaldehyde markedly enhances the formation of metalic silver from ammoniacal silver ion. The rate of silver reduction was determined with different histones by spectrophotometric measurement of colloidal silver stabilized in solution, and the apparent reactivity thus determined was found to be in the decreasing order of H1 greater than H2B greater than H2A greater than H3 greater than H4. Involvement of lysine residues was suggested since this order coincides with that of lysine content of these histones. However, the exceptionally high reactivity of histone H1 can be explained only when greater contribution of clustered lysine residues is assumed. Amino group modification and tryptic digestion studies of H1 corroborated this assumption.